I. Field of the Invention
This invention relates generally to cardiopulmonary performance analyzing equipment, and more particularly to a method and apparatus for automatically and dynamically aligning respiratory flow measurements with the measurements of individual gas concentrations of O.sub.2, CO.sub.2, etc., in the respiratory gas mixture, irrespective of changes in flow rates in the system's sampling line.
II. Discussion of the Prior Art
In the Anderson et al. U.S. Pat. No. 4,463,764 there is described a cardiopulmonary performance analyzing system for real-time, breath-by-breath acquisition, analysis and display of an individual's cardiopulmonary related parameters. In this system, a mouthpiece member containing a pneumotach is coupled to a differential pressure transducer allowing inspiratory and expiratory flow measurements to be taken. Also coupled to the mouthpiece member is a sample line leading to one or more gas analyzers which are used to measure the percentage concentration of particular gases, such as O.sub.2 and CO.sub.2 contained in the sample being drawn through the sample tube by a vacuum pump or the like. Signals from the gas analyzers are then fed into a computer-based waveform analyzer along with the flow information from the pneumotach and differential pressure transducer. While not discussed in the Anderson et al. '764 patent, it is also common practice to include a particulate filter in the sample line.
The waveform analyzer in the Anderson et al. patent not only provides for the sampling of the various analog input parameters and converting them to digital signals but it also adjusts the phase shift of the dynamic signals pertaining to the individual gas concentrations so that they will be properly aligned, time-wise, so as to be correlated with the flow. Because proper phase alignment is dependent upon the transit time of the flow sample in the sample line and because the sample flow in the sample line does not always remain constant, it is somewhat difficult to maintain precise time-wise alignment between respiratory flow and the percentage concentration of respiratory gas waveforms over prolonged periods. For example, the system may be calibrated at the onset of a patient test, but in the course of the test the particulate filter in the sample line may be affected by moisture particles, causing a variation in the sample flow rate as it becomes loaded. Also, there can be variations in the pump of the vacuum pump used for drawing the sample through the sample line. Accordingly, a need exists for a cardiopulmonary performance analyzer that allows intra-test adjustment for variations in the transit time of gas samples in arriving at the phase delay between the outputs from the gas analyzers and the respiratory flow measurement at the particular sample time.
It is accordingly a principal object of the present invention to provide a method and apparatus for dynamically aligning measured respiratory flow with O.sub.2 and CO.sub.2 signals to compensate for changing sample line flow rates.
Another object of the invention is to provide a method and apparatus for precisely determining the phase delay between the time that a sample is introduced into the sample line and the time in which the several respiratory gas analyzers generate their outputs indicative of the percentage concentration of the particular gas in the sample